Describing the harsh routine of preparing for a show's production, a glaring, growling, harsh and demanding Marsh paces back and forth in front of the lucky chorines and smokes nervously. In his vicious, bellowing voice, he ferociously delivers a dyspeptic pep talk and verbal lashing:

All right, now, everybody. Quiet, and listen to me. Tomorrow morning, we're gonna start a show. We're gonna rehearse for five weeks and we're gonna open on scheduled time. (He brandishes his cigarette) - And I mean scheduled time. You're gonna work and sweat and work some more. You're gonna work days and you're gonna work nights. And you're gonna work between time when I think you need it. You're gonna dance until your feet fall off and you're not able to stand up any longer. BUT five weeks from now, we're going to have a show! Now, some of you people have been with me before. You know it's gonna be a tough grind. (He warns some more with waves of his cigarette) It's gonna be the toughest five weeks that you ever lived through. Do you all get that? Now anybody who doesn't think he's gonna like it had better quit right now. What do I hear? Nobody? Good. Then that's settled. We start tomorrow morning.

The next morning during practice, the cast is rehearsing an insipid song in Act 1, "It Must Be June." As the group of chorines begin dancing, Marsh's main star, an atonal Dorothy Brock rehearses her lines backstage with the playwright: "Things can never be the same now." When Marsh intervenes during their squabble about her flat delivery, Dorothy repeats her line, now much more emotionally. With an idiotic grin on his face, Abner also repeats the line: "Things can never be the same now," implying something sexual about their relationship.

Billy checks in on Peggy who wears a jumpsuit with fluffed-out sleeves: "You're doin' fine for a beginner." His brief courting attempt is jarred when rival Terry (Eddie Nugent) cuts in and drags Peggy away for personal practice:

Terry: Listen sister. I'll show you those taps. Come on with me.
Peggy: Taps? Say, I can do a tap-dance on my ear. What bothers me is that routine.
Terry: Then I'm just what the doctor ordered. Come along.

The piano player who watches them sees through to the sexual economics of the mentoring:

Piano player: You kids are gluttons for punishment.
Peggy: Mister, mister, well this boy is showing me.
Terry: I was just trying to make her...
Piano player: Trying to make her is right.

In the rehearsal's climax, a slow, sluggishly-torturous rendition of "It Must Be June," Dorothy appears in Billy's arms center-stage. Marsh fidgets with the script and looks away disinterested. During their paces, Lorraine and Annie continue to share blunt wisecracks:

Lorraine: ...I always said (she) was a nice girl. And she's so good to her mother.
Annie: She sure is. Do you know that she makes forty-five dollars a week and sends her mother a hundred of it?

As Lorraine is swung in the arms of two straight-faced chorus boys, she is goosed twice, and she snaps back:

You've got the busiest hands!

Marsh stands and bellows toward the production as it ends (Billy and Dorothy emerge from under hand-held bowers):

Marsh: Wait a minute! Wait a minute!...It's out. That'll be about enough of that - it smells!
Songwriter Harry Warren (playing himself): You mean you don't like this number?
Marsh: Sure I like it. I've liked it since 1905. What do you think we're putting on? A revival? It's out, the whole number.

At the Stage door Entrance of the 42nd Street Theatre during a one-hour lunch break, Dorothy Brock rendezvouses with her love interest, a waiting Pat Denning (George Brent), her former vaudeville partner. After giving him a discreet signal, they both climb into the back-seat of a taxicab for a clandestine meeting. He complains about waiting and sneaking around to see her:

Pat: Well dear, it's just that I'm getting tired of this hiding in doorways and sneaking in and out of places. Always keeping under cover. Why, I'm beginning to feel like a criminal.
Dorothy: (She kisses him enthusiastically) There, there's nothing criminal in that, is there?

The director's hand places a cut-out of performers in a model of the musical set. Producers Jones and Barry warn: "It looks like we're in trouble." Abner Dillon, who is "puttin' up the bankroll" for the show (vilified by Barry as a "Bulgarian boll weevil mourning its first-born"), may withdraw his financial support because "dear little Miss Brock is two-timing Abner right under his very nose...with a fellow who used to be her partner for years when she was in vaudeville." Marsh wastes no time in responding, fearing that the show might fold before it even opens:

Try to apply a little dole to the problem. Give the Romeo a hundred bucks. Get him out of town. Get him out of the way...No vaudeville chump is gonna ruin my show.

Determined to not let anything interfere with the show ("What's one man's meat is another man's Murphy. I told you what this show means to me."), Marsh telephones one of his "friend(s)...from downtown," gangster Slim Murphy (Tom Kennedy) at a raucous pool hall, asking him to strong-arm Denning, lest Abner find out and withdraw backing:

Marsh: Well, listen to me and get this. It seems there's a certain guy by the name of Denning, D - E - N ...
Murphy: ...N - I - N - G. I got ya, Mr. Marsh. Sure...It's as good as done. And say, you ain't gonna forget me on ducats for the new show, are ya? That's a boy. OK.

At a restaurant table covered with a red-checked tablecloth, Denning and a fur-coated Brock finish dinner. ("You're Getting to Be A Habit with Me" plays in the background.) Depressed about relying on her wealth, he refuses her offer to pick up the bill, but she is grateful for all he taught her:

Pat: Not another nickel, honey. No, just can't be done anymore, that's all.
Dorothy: Oh Pat, don't be silly. We've always shared and shared alike, haven't we?...Well what's come over you?
Pat: Oh, getting a sudden attack of manhood possibly...Let's quit kidding ourselves. Why I'm getting to be a regular anchor around your neck...
Dorothy: But darling, I owe everything I am to you. With you and that little act of ours that started me in all this. Why you trained me and coached me and taught me all I know. I was pretty dumb too. I haven't forgotten...
Pat: You've gone ahead, you've earned it. I've stayed behind where I belong, I guess.
Dorothy: You've deliberately kept yourself behind.
Pat: Nothing of the sort. Now they wanted you. They never wanted me. (Dorothy looks down) Besides darling, I've been getting myself some education. And I've discovered they have a name for a man who doesn't work and who accepts money - from a woman. It isn't a very nice name, Dot.

The next rehearsal of tap dancing is for the production number "42nd Street." In a realistic vignette of backstage life, Marsh frantically screams and pounds out the beat: "Watch that tempo! Watch it, will you?! Come on, get your feet off of the floor. Get 'em up! Faster, faster! Come on, faster! Faster! Stop it! Stop it! Stop it! IT'S BRUTAL." In the acrid, perspiring air of the stage, Marsh berates the dancers, his voice cracking and pleading:

Oh! May I remind you that Pretty Lady's out-of-town opening is not far away! It's been advertised as a musical comedy with dancing. If it isn't asking too much, will you please show me a little? All right, once again, give it something!

Out in the audience, even Abner is deathly bored with the gyrating legs:

After three weeks of this, a leg ain't nothin' to me but something to stand on.

In the next practice segment however, Abner's attention is inspired by Dorothy's "You're Getting to Be a Habit With Me" number as she hops up on the piano: "Doesn't she sing gorgeous?" Barry reminds him of the bottom line: "Seventy thousand bucks." More rehearsals capture the agonizing, endless work of churning out the performances. A super-imposed clock rotates above the dancers. As the pace is intensified when slave-driving choreographer Andy barks out at the chorus, Peggy Sawyer becomes dizzy, faints, and collapses. Marsh fears any interruptions and shouts for them to continue, screaming at Billy to remove her so that everyone can return to the rehearsal:

This is a rehearsal, not a rest cure.

Peggy is carried to the backstage stagedoor entrance, and given a glass of water by Billy. After everyone returns to rehearsal, Peggy is befriended by a flirtatious Pat Denning (waiting for Dorothy), who pre-cognitively foretells Peggy's future star status. Their innuendo-filled dialogue in a doctor-patient fantasy is remarkably, but mildly daring:

Peggy: I guess I'm all right now. (She rises but is unsteady on her feet)
Pat: Bad guess. You'd better sit down again. Listen, you let me play now.
Peggy: What would you suggest, doctor?
Pat: A little fresh air and conversation.
Peggy: What about Mr. Marsh?
Pat: Oh, never mind Mr. Marsh. Let's sit this dance out, huh?
Peggy: And if I lose my job?
Pat: If you do, there just won't be any show!
Peggy: Anyway, it's a nice idea.
Pat: Say listen, I've got a lot of nice ideas. What I need are ears to spill them in.
Peggy: Well, won't mine do?
Pat: Yes, you know, I think they do very, very nicely. When are your ears available?
Peggy: Oh, at all sorts of odd hours. You must look them up sometime.
Pat: Remind me to tell you I think you're swell.
Peggy: Well thanks doctor. Your prescription was great. Now for the workshop.

Dorothy is escorted out to dinner by an eager Abner, and as Pat trails behind them, she signals for him to leave them. Now on the rebound, Denning returns inside and picks up a coquettish Sawyer as she leaves the night's rehearsal, claiming that he is now the "patient" rather than the "doctor." In need of advice, and with a pick-up line, he encourages her to go to dinner:

Pat: What do you advise for a man who's both hungry and lonesome and who hates to eat alone?
Peggy: Company!
Pat: Excellent. Did you have any one in mind in particular?
Peggy: Did you?
Pat: Will you?
Peggy: I'm very much afraid I will.

After their evening together and he has walked her back to the front steps of her boarding house, Denning is confronted by Slim Murphy and told: "This guy Denning's a pretty wise mug but he ain't wise enough. If he don't lay off that Dorothy Brock dame, it's gonna be just too bad for Denning, get me?" Denning is punched and kicked ("That's so you don't forget") and left laying on the sidewalk. Peggy brings him into her upstairs room and takes care of his bruises. They are discovered, with Pat on Peggy's bed, by a disapproving Irish landlady who criticizes the impropriety of her entertainment of a male guest: "What kind of a house do you think this is?" As an innocent Peggy is lectured and evicted, another female boarder across the hallway (behind the landlady) helps a suspendered boyfriend tiptoe out of her room and down the stairs.

With no other choice but a "park bench," Peggy must accompany Denning to his place, the first time she has ever been alone in a man's place. As he carries her luggage into his apartment, syrupy-sweet music plays behind their entrance. As she adjusts her collar in a mirror, the suave playboy switches off the lights - Peggy jumps suspiciously and realizes her predicament. When Peggy sinks into his leather couch, he brings them shots of illegal booze to ready her for seduction:

Just a couple of rose-colored glasses. Let's try them on and see how the world looks.

After one sip, Peggy pleads that she is tired from the dance rehearsals, and she begins nodding off: "You hop and step as many miles as I have today and you'd be tired too." Thwarted by her fatigue, he scoops her out of the sofa and carries her to his back bedroom. She protests his sudden movement: "Hey Pat, what are you doing? Please put me down, please Pat! Please put me down." As he places her on his bed, she bounces up into a sitting position - quickly, he reassures her:

Now you go right to sleep youngster. What you need is a good night's rest.

After he has shut her door, she contemplates for a moment, and then reaches over to the door. In a dollying close-up, she locks the door with the key.

Long all-night dancing rehearsals continue with Peggy helping Billy with his dance steps: "Just pick 'em up and lay 'em down." At one point, all the dancers unimaginatively jump up and down in tandem. One of the chorus boys criticizes Billy from afar: "Get a load of the juvenile. Didn't know we had elephants in the show." Marsh adds his insults to Billy's tapping:

Someday, some director's gonna jail you for taking money under false pretenses. You're supposed to be a dancer. All you need is a couple of license plates and you look like a Model T Ford.

Denning's and Peggy's friendship grows. When Dorothy stops in Denning's apartment for a visit, she notices evidence of another visit: "Oh, tea for two!" But he covers up: "Yeah, friend of mine dropped in." As Dorothy's signature song plays in the background, she wonders about his love, and then spots his packed luggage. Denning tells her he is leaving New York "for a stock job in Philadelphia," to find his own success. In a heart-to-heart talk, Dorothy has figured out the problems between them. Their breakup becoming inevitable, she encourages him to independently venture out on his own and work with gainful employment in Philadelphia until they can meet openly:

Dorothy: We've grown too necessary to one another. You've been content to rest in the shadows while I basked in the spotlight, simply because it's held us together. My success has been your failure.
Pat: No dear. My failure, as you put it, has been my happiness.
Dorothy: But darling, you're capable of such great things. You're not a quitter. I've only wanted to help you, but I've been hurting you instead. I realize that now. That's why I came here this morning. Pat, we're not going to see each other for a long time. No more doorways. No more secret meetings. You're going out on your own and make a success.